Linear actuators are used in many different applications. In the automotive field, linear actuators are used, inter alia, for positioning headlights in the horizontal and vertical planes, as described in publications U.S. Pat. No. 6,641,292, US 2005/0046291, and US 2005/0174796. The devices described in these publications include a stepping motor driving a screw-nut system for the linear displacement of a screw shaft. With the stepping motor, it is possible to displace and position the screw shaft rapidly, with few mechanical parts and by using a relatively simple control.
In the field of automobile headlight adjustment, it is important to have a high performance, reliable and dynamic screw-nut system, while using compact, lightweight and economical components. Components used in automobiles should moreover perform even under extreme environmental conditions, in particular for ranges of temperatures from −40° C. to 120° C.
In conventional actuators, the torque to be exerted by the rotor on the member to be displaced decreases when the temperature decreases, because of the increase of frictional forces in the bearings positioned between the rotor and the stator. This is due to the increase in the viscosity of the lubricants present in the bearings. Although certain lubricants have an acceptable viscosity at a very low temperature, the latter are either expensive or unacceptable for ecological reasons.
Another disadvantage of the aforementioned conventional actuators is the mechanical play that occurs when the displacement direction of the linear member is changed, this play being detrimental to the positioning accuracy of the headlights, as well as causing vibration and wear of the components.
US 2006/0108885 A1 discloses a motor with ball bearings, intended for consumer electronics applications. This motor is however not suitable for a linear actuator application in the automotive field because of the instability of the bearings. Indeed, the balls are housed in cavities of parts assembled in the housing of the motor, the balls being in abutment against an essentially spherical surface at the ends of the axis of the motor, but in a separable manner. In the case of vibration or shock, the interface between the balls and the ends of the axes may separate and even be fouled and worn, thereby leading not only to the possibility of radial vibration of the motor, but also to decentralization of the rotor. Moreover, the shape of the housing in which the balls are mounted, is sensitive to wear and positioning of the balls and requires accurate manufacturing and mounting tolerances.